Disturbances which occur in a data channel can be categorized as either additive or multiplicative. An undesirable additive disturbance signal is simply added to the information (data) signal. An undesirable multiplicative disturbances or data density change causes a modulation of the data signal.
In the data channel, where the signal sensing transducers, for example magneto-resistive (MR) sensors are exposed to the air in the air bearing surface of a slider assembly and a rotating magnetic disk, additive disturbances can occur due to physical frictional contact of the sensors(s) with the moving recording surface of the disk. The disturbances result from the friction-generated elevated temperature (up to 120.degree. C.) at the contact spot. This friction-generated elevated temperature produces a small yet sudden increase in temperature of the MR sensor, for example in the order of 1.degree. C. averaged over the entire sensor within about 50 to 100 nanoseconds. Due to the nonzero temperature coefficient of resistance of the MR sensor (approximately 0.003/.degree. C. for permalloy), the sensor resistance will increase with this sudden temperature rise. The heat conducted into the MR sensor from the hot spot will diffuse slowly to the environment of the sensor, causing the resistance increase to decay slowly to the original value. Typically, a drop to about 30% of the thermally induced resistance change will occur in 1.5 to 5 microseconds (ps).
Such a combination of signals and disturbances cause many problems with signal detection in the data channel. The automatic gain control (AGC) circuit in the channel may fade out quickly during the transient and recover only slowly. Even if an AGC circuit were to accommodate the disturbed signal, the thermal transient would still result in a peak shift, for example, the data signal is differentiated for peak detection, and as a result of this the thermal transient will also be differentiated. This leads to an extra zero crossing and a shift of the zero crossing level after the thermal transient.
FIG. 1 illustrates the magnetic signal or the information signal 2 to be detected by the MR sensor without the disturbance.
In FIG. 2, a disturbance signal 4 is illustrated without the information signal 2, which may be caused by the physical frictional contact with the sensor with the moving recording surface of the disk.
FIG. 3 illustrates two signals, namely the disturbance signal 4 plus the information signal 2 to form the input signal 6 detected by the magneto-resistive head from the MR sensor and additively combined.
Several devices illustrated in the below references are attempted unsuccessfully to detect the information signal 4 during time period of a disturbance signal 4.
U.S. Pat. No. 3,566,281 discloses positive and negative peak detection, which are offset by a constant voltage and averaged and subtracted from the delayed input signal.
U.S. Pat. Nos. 3,473,131 and 4,356,389 are other patents which do not solve the problems of additive disturbances.
U.S. Pat. No. 4,914,398 discloses positive and negative envelope detectors and a buffer which interconnects the detectors with a nonlinear signal-adaptive filter. However, the device of this patent introduces phase distortion which is caused by the low pass filter. The sharp disturbance signal 5, as illustrated in FIG. 3, tends to be an abrupt change from the steady state, and the filter response makes it difficult to respond to these abrupt changes. In consequence, it is difficult to detect high frequency responses. Thus, it is difficult to maintain group delay ##EQU1## due to the phase distortion and ripple rejection simultaneously, inspite of the adaptive low pass filter.
In order to eliminate the disturbance signal 4 one of the intended requirements is to determine when the disturbance signal occurs, namely in FIG. 3, at t.sub.1.
One circuit to detect when the sharp disturbance signal occurs, namely at t.sub.1 is a threshold detector in the form of a comparator. Such a detector is illustrated in FIG. 4.
The comparator 100 compares the output of amplifier 104 with the output of voltage source 102. The voltage source 102 is set at a threshold voltage. The comparator 100 outputs a signal when the output of the amplifier 104 exceeds the threshold voltage. If the threshold voltage is sufficiently high, for example 20% of the sharp disturbance signal 5 over the zero level of the information signal 2. When the sharp disturbance signal 5 exceeds the threshold voltage, a signal, for example a TA signal, output from the comparator 100, indicates the occurrence of the sharp disturbance signal 5.
FIG. 5 illustrates a MR head 130 connected to a preamp 100 and connected to a read channel circuit 120. The TA signal is activated by the sharp disturbance signal 5.
The problem with this configuration is that the circuit does not take into consideration a DC offset of the information signal 2. The amplifier which may be a preamplifier may cause the information signal 2 to be biased with a DC offset voltage. This DC offset voltage effectively raises the information signal 2 to a point above the threshold voltage causing a false indication of the sharp disturbance signal.
Thus, it is desirable to eliminate the disturbance signal offset voltage from the information signal 2 to provide for more accurate detection of the sharp disturbance signal 5.